Air-breathing direct methanol fuel cell with metal foam current collectors

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of provisional U.S. patent application Ser. No. 60/264,137, filed Jan. 25, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to polymer electrolyte membrane fuel cells suitable for direct oxidation of liquid fuels. More particularly, this invention relates to direct methanol fuel cells which eliminate the need for pumps, blowers and humidifiers in order to operate.

2. Description of Related Art

Direct methanol polymer electrolyte membrane fuel cells are prime candidates for both vehicular and stationary uses due to their inherent simplicity (no external reformers) and potential high energy densities (liquid fuels). In addition, direct methanol polymer electrolyte membrane fuel cells have the potential for replacing rechargeable batteries due to the possibility of a zero recharge time. However, the current state of the art in direct methanol polymer electrolyte membrane fuel cells requires external means, such as pumps and blowers for introducing reactants into and removing reaction products from the fuel cell. For example, U.S. Pat. No. 5,573,866 to Van Dine et al. teaches a polymer electrolyte membrane fuel cell which directly oxidizes liquid methanol fuel that is fed into the anode chamber from a liquid methanol storage container. The liquid methanol is mixed with water in the anode chamber. Some of the methanol and water cross over the membrane into the cathode chamber and into a process air stream. The methanol and water are removed from the cathode chamber by evaporation into the process air stream, which is then directed into a condenser/radiator. The methanol and water vapors are condensed in the condenser/radiator, from whence the condensed water and methanol are returned to the anode chamber of the cell. The evaporating cathode process air stream, which is provided to the cathode chamber by means of a fan, provides oxygen for the fuel cell reaction, and also cools the cell. See also U.S. Pat. No. 4,390,630 to Kawana et al. which also teaches a methanol fuel cell. It will be appreciated that, to the extent direct methanol fuel cells require the use of external devices for operation, such as fans, blowers and humidifiers, they are not suitable for use in vehicular applications or as a replacement for rechargeable batteries.

SUMMARY OF THE INVENTION

Accordingly, it is one object of this invention to provide a polymer electrolyte membrane fuel cell suitable for generating electricity by direct oxidation of methanol that does not require the use of external means for providing oxidant to the fuel cell and/or removing water from the fuel cell. More particularly, it is one object of this invention to provide an electricity-producing, self-sustaining direct methanol polymer electrolyte membrane fuel cell.

It is another object of this invention to provide a method for generating electricity by direct oxidation of methanol that eliminates the need for auxiliary equipment such as fans, blowers and/or humidifiers for delivery to the fuel cell and removal from the fuel cell of reactants and reaction products, respectively.

These and other objects of this invention are addressed by an apparatus for generating electricity comprising an enclosure suitable for containment of a liquid hydrocarbon fuel and at least one air-breathing polymer electrolyte membrane fuel cell unit disposed in a wall of the enclosure with an anode side facing an interior of the enclosure. The at least one air-breathing polymer electrolyte membrane fuel cell unit comprises a polymer electrolyte membrane having an anode side and a cathode side, an anode electrode disposed on the anode side of the polymer electrolyte membrane and a cathode electrode disposed on the cathode side of the polymer electrolyte membrane. A porous metal cathode current collector having a centrally disposed active region is disposed on the cathode side of the polymer electrolyte membrane and a porous metal anode current collector is disposed on the anode side of the polymer electrolyte membrane. In accordance with a particularly preferred embodiment of this invention, the porous metal cathode current collector and the porous anode current collector are made from a foam metal. As will be shown, critical to the operation of the polymer electrolyte membrane fuel cell of this invention without the need for auxiliary pumps, fans, humidifiers, etc. are the porosity and pore sizes of the porous current collectors.

In accordance with one embodiment of the method of this invention, methanol is supplied through a porous anode current collector of a polymer electrolyte membrane fuel cell to the anode electrode and oxidant is supplied at substantially atmospheric pressure by means of diffusion through a porous cathode current collector to the cathode electrode of the polymer electrode membrane fuel cell. The methanol is directly oxidized in the fuel cell, resulting in the generation of water, carbon dioxide and electricity. The water generated in the fuel cell diffuses out of the fuel cell through the porous cathode current collector. In operation, there exists substantially no pressure differential between the anode side and the cathode side of the polymer electrolyte membrane fuel cell.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings, wherein:

FIG. 1 is a diagram showing the components of a direct methanol fuel cell in accordance with one embodiment of this invention;

FIG. 2 is a diagram of an air breathing direct methanol fuel cell in accordance with one embodiment of this invention;

FIG. 3 is a diagram showing cell voltage and power density of the direct methanol fuel cells of this invention;

FIG. 4 is a diagram of a direct methanol fuel cell pack in accordance with one embodiment of this invention;

FIG. 5 is an exploded view of a direct methanol fuel cell from the direct methanol fuel cell pack of FIG. 4; and

FIG. 6 is a diagram showing a possible reaction scheme for formation of a chemical barrier which reduces methanol crossover.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The apparatus for generating electricity in accordance with this invention is an air-breathing direct methanol fuel cell unit and air-breathing direct methanol fuel cell assembly or pack. As used herein, the term “air-breathing” refers to the ability to take in air or other oxidant at substantially atmospheric pressure without employing auxiliary pumps, fans, humidifiers, etc. As shown in FIG. 1, each direct methanol fuel cell in accordance with this invention comprises a membrane-electrode assembly or MEA <b>15</b>, which membrane-electrode assembly comprises a polymer electrolyte membrane <b>17</b> disposed between an anode electrode <b>18</b> and a cathode electrode <b>16</b>, an anode current collector <b>11</b> and a cathode current collector <b>10</b>. The key to this invention is the use of special metal foams as the current collector at the cathode electrode. In addition to functioning as a current collector, the special metal foam also acts as an air diffuser. In particular, the cathode current collector <b>10</b> acts as an air diffuser through which the amount of air required to operate the cell is delivered to the cathode electrode <b>16</b> and product water generated by the cell is removed. The cathode current collector <b>10</b> and the anode current collector <b>11</b> preferably are made of porous metal or metal alloy foam materials having a porosity preferably in the range of about 50% to about 95%. The pore sizes are in the range of about 10 to about 80 PPI, which pore sizes are critical to limiting methanol crossover. In accordance with one preferred embodiment of this invention, the current collectors are made of a porous material selected from the group consisting of porous Hastalloy C, nickel foam and stainless steel foam (available from Porvair, Hendersonville, N.C. 28792). These materials provide little resistance to the transfer of methanol solution to the anode electrode and air breathing to the cathode electrode. In addition, these materials are both corrosion resistant and electrically conductive.

In accordance with one embodiment of this invention, the peripheral areas of the foam current collectors are densified and sealed using commercially available silicone to form a gasket area <b>20</b>. The gasket area <b>20</b> generally conforms to the area of the gasket <b>12</b>, which provides sealing around the periphery of the membrane electrode assembly <b>15</b>. One suitable material for producing gasket <b>12</b> is a commercially available ethylene styrene copolymer polymer, such as SENFLEX® T-1210 and SENFLEX® T-650 available from SENTINEL Polyolefins L.L.C., Hyannis, Mass. 02601. In accordance with one preferred embodiment of this invention, the anode electrode <b>18</b> comprises an anode catalyst comprising 40% Pt—Ru (1:1 molar ratio) on carbon black, which catalyst is CO-tolerant and the cathode electrode <b>16</b> comprises a cathode catalyst which is about 40% Pt at a loading of about 0.5 to about 1 Pt mg/cm².

In accordance with one embodiment of this invention, the electrodes <b>16</b>, <b>18</b> are gas diffusion electrodes which are made using carbon cloth coated with a layer of carbon black and TEFLON emulsion. The carbon cloth is not teflonized, but it is sintered to be hydrophobic. The carbon black layer comprises about 30% by volume TEFLON emulsion so as to be hydrophobic. This gas diffusion electrode has a good and stable hydrophilicity/hydrophobicity ratio for the application in the direct methanol fuel cell of this invention. Details for producing suitable gas diffusion electrodes can be found in pending U.S. patent application Ser. No. 09/768,531, which is incorporated by reference herein in its entirety.

The current collector materials are selected based upon their conductivity, corrosion rate and porosity. The metal foam is pressed around the periphery to leave a centrally disposed active area <b>19</b>. This pressed area <b>20</b> is used for gasketing. When assembled, the direct methanol fuel cell of this invention comprises a membrane electrode assembly <b>15</b> sandwiched between two metal foam layers <b>10</b>, <b>11</b>, making assembly simple, because the metal foam layers function as a current collector, MEA supporter and, in the case of the cathode current collector <b>10</b>, an air diffuser.

FIG. 2 shows an exemplary direct methanol fuel cell assembly comprising eight direct methanol fuel cell units, only four of which are shown (<b>21</b>, <b>22</b>, <b>23</b> and <b>24</b>). The direct methanol fuel cell assembly comprises a housing <b>32</b> in which the methanol solution is disposed. The walls <b>25</b>, <b>26</b>, <b>33</b>, <b>34</b> of the housing form at least one opening sized to accommodate at least one direct methanol fuel cell unit. A methanol solution is disposed within the interior of the direct methanol fuel cell assembly at least at a level sufficient to contact the active region of the anode electrodes of the fuel cell units. As shown in FIG. 2, the housing <b>32</b> forms openings <b>29</b>, <b>30</b> through which methanol solution is introduced into the housing <b>32</b> and product gases are exhausted from the housing. In accordance with one preferred embodiment of this invention, the methanol solution comprises at least one addictive suitable for preventing impurities that may be present in the solution from passing into the active regions of the fuel cell units. One such additive suitable for this purpose is iso-propanol, available from Aldrich Chemicals.

The idea is to add additives in the methanol solution. A condensation reaction of an alcohol with an acid to produce an ester and water; e.g. CH₃OH+C₆H₅COOH⇄CH₃OOCC₆H₅+H₂O. The reaction is an equilibrium and is slow under normal conditions, but can be speeded up by the addition of a strong acid catalyst. Nafion is a fluoro-sulfuric acid, which reacts with methanol to form an ester. This reaction is slow; however, in the Nafion membrane, due to the high concentration of acid, the reaction could be fast. In the direct methanol fuel cell, methanol crossover is one factor that reduces the cell performance. That methanol reacts with Nafion and stays in the Nafion are other reasons to increase the cell IR. The condensation reaction is inevitable in the direct methanol fuel cell; however, we use the condensation reaction to form a chemical barrier, which reduces methanol crossover. A reaction scheme for chemical barrier formation is shown in FIG. <b>6</b>. IPP is a three-carbon molecule, which is very difficult to oxidize to carbon dioxide and water. This “T” shape chemical barrier occupies sites, on which methanol molecules may sit. Other molecules, such as ethylene glycol, butanol, etc. are also candidates for the chemical barrier. However, physical properties, such as viscosity and solubility must be considered. The properties of the additives must be stable, not poison the catalysts, and also not restrict proton movements. The chemical barrier has equilibrium in the methanol solution, but is not consumed. Therefore in the direct methanol fuel cell, the methanol is the fuel to be consumed.

EXAMPLE

A direct methanol fuel cell assembly as shown in FIG. 2 was made having an active area of about 5 cm², providing about 1 W at 60° C. Performance of the assembly was 0.38 V/cell at 60 mA/cm². At room temperature, the performance of the assembly was 0.38 V/cell at 10 mA/cm². This direct methanol fuel cell assembly was able to run a 3 V transistor radio and is capable of running other low power systems as well.

FIG. 3 is a diagram showing the polarization curve of a direct methanol fuel cell assembly or pack at room temperature in accordance with one embodiment of this invention. The cell pack performed at about 150 to 200 mW. Performance of the cell pack is highly dependent upon liquid temperature and methanol concentration. The temperature of the methanol solution is preferably in the range of about room temperature to about 100° C. while the methanol concentration in the solution is in the range of about 2% to about 20% by volume. The higher temperatures increase cell performance, as exemplified by a pack at 60° C. in which the pack power reaches 1 W. Methanol crossover is controlled by adjusting the methanol concentration and the cathode current collector pore size.

FIG. 4 is a diagram of a cylinder-shaped direct methanol fuel cell pack <b>40</b> in accordance with one embodiment of this invention having twelve direct methanol fuel cell units <b>55</b>, <b>56</b>, <b>57</b>, one of which is shown in an exploded view in FIG. <b>5</b>. The “striped” fuel cell unit is wrapped on a perforated plastic cylinder. In this embodiment, the anode current collector <b>54</b> was made with a perforated metal while the cathode current collector <b>53</b> was made with metal foam. As shown in the exploded view in FIG. 5, the fuel cell units of the fuel cell pack are separated by plastic elements <b>51</b>, <b>52</b>. It will be apparent to those skilled in the art that the simplicity of the air breathing direct methanol fuel cell units of this invention provides the possibility for fuel cell packs having a variety of shapes, in addition to the rectangular and cylinder-shaped packs described herein, and all such shapes are deemed to be within the scope of this invention.

While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for the purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of this invention.